ABSTRACT The elicitation of potent and broad HIV-1 neutralizing antibodies (bNAbs) by immunization has been one of the major goals of HIV-1 vaccine research since the beginning of the HIV/AIDS epidemic. During the past decade, significant technical and conceptual advances have enabled the isolation and detailed characterization of a plethora of new bNAbs from HIV-1-infected subjects. The structural characterization of such antibodies, combined with information on their ontogenies, has vastly improved our understanding of how such antibodies are generated during natural infection, leading to new hypotheses regarding how to elicit them by immunization. Our HIVRAD Program grant aims at testing novel reagents and prime-boost immunization schemes to elicit VRC01-class bNAbs, which are among the most broad and potent HIV-1 neutralizing antibodies known and display impressive protective potential in animal studies. However, the development of VRC01-class bNAbs by immunization will necessitate overcoming several obstacles. A successful immunization scheme will likely require, at a minimum, the availability of novel immunogens to initiate and guide the bNAb maturation process, the development of strategies that will minimize the expansion of competing off-target B cells, the development of optimal ?boost? Env immunogens to guide the appropriate antibody maturation, and the supply of sustained T helper responses. Here we propose to test concepts, not tested previously, that we have developed to directly address these issues. One new strategy we will evaluate is based on the use of anti-idiotypic monoclonal antibodies (aiMAbs) we generated against germline VRC01- class BCRs. Our preliminary data suggest that these aiMAbs specifically expand naive B cells that express germline VRC01-class B cell receptors (BCRs) and that these cells enter the germinal center (GC) reaction and expand even further upon Env immunization. One obvious advantage is a greater expansion in GC of desired B cells over ?off target? B cells, thus improving our chance to induce the correct set of somatic hypermutations during the boost immunizations. Thus, immunizations with aiMAbs will be followed by booster immunizations with an Env specifically designed to engage germline VRC01-class BCRs (426c Core) and then with Envs expressing key steric blocks for VRC01-class antibodies. In this regard we highlight the fact that since our initial submission we generated new information which indicates that the 426c Core elicits antibodies that bypass glycans on the conserved N-linked glycosylation site N276, which are one of the major obstacles preventing germline VRC01-class antibodies from becoming broadly neutralizing. We will test our proposed strategy by an iterative approach with well-integrated experiments. The preliminary data we present support our overall approach.